1. Field of the Invention
The present invention relates to systems and methods for free-space optical communication networks and to a system and method for controlling the power of a laser used in such a network.
2. Description of the Related Art
Currently, the primary method for data transmission between remote locations utilizes wired lines or fiber optic cables. Some of the costs associated with this method are due to the expense in obtaining rights-of-way for the cable runs as well as installing the cables by burying or hanging. While this method has proven successful where great distances separate two locations, it is prohibitively expensive between locations that are within close proximity to one another.
The dramatic growth in the demand for broadband services and the time and expense associated with deploying traditional wired lines or fiber optic cables have led to the development of new wireless broadband access technologies. One of these new wireless technologies employs a Light Amplification Stimulated Emission of Radiation (laser) beam to transmit information. Such a system may consist of at least 2 optical transceivers accurately aligned to each other with a clear line-of-sight to deliver the information using such a laser beam.
However, when the communication laser beams are present in a location accessible by people, laser safety becomes an important issue. Unlike light produced by a common lamp or the sun, laser light is not divergent and often emits radiation within a narrow band of wavelengths to form a monochromatic light. Furthermore, because this laser light is coherent and non-divergent, it is easily focused by the lens of a human eye to produce images on the retina with greater intensity than is possible with these other common sources of light.
Safety guidelines do exist for the use of lasers. For example, such guidelines are promulgated by the International Electrotechnical Commission (EC) based on a maximum permissible exposure (MPE) level. If one were to apply such a standard, a maximum power level could be predicted (known as an Accessible Emission Limit (AEL)) that would make the communication laser beam eye-safe to a viewer, known as a class 1 laser system in the EC standard. However, to establish and maintain a high-bandwidth connection, the lasers used in such systems may transmit at power levels that exceed the class 1-power levels designated by these laser safety guidelines.
Therefore, there is a need for a system and a method that allows the use of optical communication beams of light with adequate power to provide a robust optical link between communication terminals while minimizing safety risks to either users or a passerby. Such a system and method may maintain a signal-to-noise ratio above a desired value at the distant receiving communication terminal and under various environmental conditions that tend to degrade the signal, such as fog, smog, rain, or snow. Moreover, such a system and method could expand the permissible locations for placement of such optical transceivers to places that are accessible to humans.
Optical-to-electronic conversion of a communication laser beam is an important process. In many optical communication systems, for example, an information-bearing optical wave, after transmission through an optical link, is received by an optical detector within the transceiver. The optical detector converts the optical wave into an electrical signal for further processing. The optical detector can include a photosensor and a detector circuit coupled thereto. The photosensor, such as a photodiode, converts the received photons of the optical wave into an electrical signal. This electrical signal is in the form of photo current or a photo voltage. For a given photosensor, the design and operation of its detector circuit can be configured to enhance the advantages and suppress disadvantages of the photosensor for a specific application. For example, the detector circuit can be used to calibrate the photosensor. Once calibrated, the detector circuit can further provide an electrical bias to the photosensor to process or condition the electrical signal to produce a detector output.
Laser transmitter and detector sensitivity dynamic ranges are often mismatched. Laser transmit power control typically have a more limited dynamic range that the laser detector. When optimal weather conditions occur between a laser transmitter and a detector, the laser transmitter can oversaturate the detector. Due to the limited dynamic range of the laser transmitter, the system may be unable to reduce the laser transmitter's power to prevent oversaturation. Additionally, incident light is reflected by the receiver and in a direction towards the transmitting laser. A receiver, which is associated with the transmitting laser, may experience interference with its incoming communication beam from this reflected light.
The quality of a received signal is often degraded when a detector is not aligned with the incoming communication beam. Alignment between a transmitter and a receiver is often performed using a signal transmitted between the transmitter and receiver. However, the signal's power may have a wide dynamic range which is difficult to process by the receiver.
Thus there is a need for system and method which calibrates a photosensor and enhances its operational dynamic range. The system should also attenuate the power level of an incoming communication beam to prevent oversaturation of a receiver. The system should further provide an alignment signal which is effective over a wide dynamic range of incoming power levels.